As well known, the connecting rod or conrod (preform or blank) has a large end section constructed as a splittable structure intended for mounting to a crankshaft. Among various methods for constructing a conrod as such a splittable structure is a fracture-based slitting method. Various connecting-rod fracture splitting apparatus employing the fracture-based slitting method have heretofore been proposed, one example of which is disclosed in Japanese Patent No. 3642268 ((hereinafter referred to as “Patent Literature 1”). More specifically, Patent Literature 1 discloses an apparatus which expands a mandrel unit by inserting thereinto a wedge connected to a hydraulic actuator, and in which a pressure accumulator is provided in a hydraulic circuit.
As shown by the vertical axis of FIG. 6 in Patent Literature 1, the connecting-rod fracture splitting apparatus is characterized by controlling a load to be applied to the large end section stepwise to a preload load and to a fracture load. With the pressure accumulator provided in the hydraulic circuit of the apparatus, high-pressure hydraulic oil can be supplied quickly or instantaneously, so that the fracture load can be applied instantaneously.
Further, as shown by the horizontal axis of FIG. 6 in Patent Literature 1, the hydraulic actuator requires a considerable rise time because it starts operating from a rest state. Therefore, a considerable time is required before the fracturing starts, which would create an undesirable stretch in the large end section and an undesirable time difference between a left fractured portion and a right fractured portion. As a result, roughness and cracks of the fractured surface can occur, resulting in low quality of the conrod.
As a measure against the aforementioned prior art inconveniences, there has been proposed an apparatus which employs a collision method such that it can perform quick fracturing by causing collision of a mass body, as disclosed in Japanese Patent No. 2695272 (hereinafter referred to as “Patent Literature 2”). According to the disclosure of Patent Literature 2, the inconveniences of the apparatus disclosed in Patent Literature 1 can be overcome because the collision method can apply the fracture load more quickly than the hydraulic actuator. However, the apparatus disclosed in Patent Literature 2 would undesirably introduce deterioration of working environment due to large sound produced by the collision (i.e., collision sound). Thus, in recent years, there have been great demands for improvement of the working environment.